1. Field of the Invention
The present invention relates to telephone hybrid circuits and circuits for separating a receive audio signal from a transmit audio signal.
2. Background Art
Traditionally, telephone subscriber lines have been twisted pair wires. The two conductors of the twisted pair are referred to as tip and ring. Communication in both directions is provided by such lines. Since a person speaking into a telephone handset expects to hear his or her own voice in the earpiece, a telephone instrument introduces the transmitted signal into the receive circuit so that the transmitted signal will be heard along with the received signal in the earpiece. The transmitted signal heard at the earpiece is referred to as sidetone. Since sidetone is provided, it is generally unnecessary to completely separate the receive signal and the transmit signal, which are both present on the two conductor telephone line, from each other.
Unlike voice telephone instruments, data modems generally do not provide sidetone. Since the presence of the transmitted signal in the receive circuits would tend to interfere with the reception of the received signal, data modems usually provide circuits to separate the received signal from the transmitted signal. Such circuits are referred to as hybrid circuits. Hybrid circuits may be used to combine transmitted and received signals into a single communication path or to separate a single communication path into separate transmitted and received signals.
Hybrid circuits are also often used in both voice and data telephony for coupling a two conductor telephone line, which provides bidirectional communication, to two unidirectional trunk lines, one of which carries the transmitted signal, the other of which carries the received signal. Such coupling is referred to as two-wire-to-four-wire conversion (or four-wire-to-two-wire conversion), although it is not limited to two wire and four wire circuits, but may be used with other communication media, such as fiber optics, microwave and radio frequency (RF) media, and other types of media.
Early hybrid circuits relied on specially wound transformers to separate or combine the transmit and receive signals. The windings were phased so as to couple the desired signals in phase, but to couple the undesired signals out of phase. Thus, the desired signals are passed, but the undesired signals are cancelled.
An example of a transformer based hybrid circuit is illustrated in FIG. 1. The circuit includes transformer 101 having windings 103, 104, 105, and 106, transformer 102 having windings 107, 108, 109, and 110, resistor 111, and capacitor 112. A two wire transmit signal interface is provided at nodes 115 and 116. Node 115 is coupled to the first terminal of winding 103. The second terminal of winding 103 is coupled through node 120 to the first terminal of winding 104. The second terminal of winding 104 is coupled to node 116.
A two wire receive signal interface is provided at nodes 117 and 118. Node 117 is coupled to the first terminal of winding 109. The second terminal of winding 109 is coupled through node 121 to the first terminal of winding 110. The second terminal of winding 110 is coupled to node 118.
A two wire bidirectional signal interface is provided at nodes 113 and 114. Node 114 is coupled to the first terminal of winding 108. The second terminal of winding 108 is coupled through node 119 to the first terminal of winding 106. The second terminal of winding 106 is coupled to node 113.
The first terminal of winding 105 is coupled to the first terminal of winding 107. The second terminal of winding 105 is coupled through node 123 to the first terminal of capacitor 112 and to the first terminal of resistor 111. The second terminal of winding 107 is coupled through node 124 to the second terminal of capacitor 112 and to the second terminal of resistor 111.
A transmit signal applied across nodes 115 and 116 is coupled to nodes 122 and 123 by windings 103 and 105 and to nodes 119 and 113 by windings 104 and 106. Current through winding 106 also flows through winding 108 and through the two wire bidirectional interface at nodes 113 and 114. The signal across nodes 114 and 119 is coupled through windings 108 and 110 to nodes 118 and 121.
The current flowing through winding 105 also flows through winding 107 and through the balancing network comprising resistor 111 and capacitor 112. The impedance of the balancing network must be equal to the impedance across nodes 113 and 114 of the two wire bidirectional interface for proper symmetry of the circuit and cancellation of signals in the windings.
The signal at nodes 122 and 124 is coupled to nodes 121 and 117 through windings 107 and 109. Since the current flowing through winding 105, winding 107, and the balancing network comprising resistor 111 and capacitor 112 is the same as the current flowing through winding 106, winding 108, and the two wire bidirectional interface at nodes 113 and 114, the current flowing through winding 109 is equal to the current flowing through winding 110. However, due to the crossed connection between winding 105 and 107, the signal across winding 109 is out of phase with the signal across winding 110. Since the signals are equal but out of phase, they cancel one another out. Thus, the transmit signal is not present across the receive signal interface at nodes 117 and 118.
A signal applied at the two wire bidirectional interface across nodes 113 and 114 flows through windings 106 and 108. The signal at nodes 119 and 113 is coupled to nodes 116 and 120 through windings 106 and 104. The signal at nodes 114 and 119 is coupled to nodes 118 and 121 through windings 108 and 110. Current flowing through winding 110 also flows through winding 109 and through the two wire receive signal interface across nodes 117 and 118.
The signal across nodes 121 and 117 is coupled to nodes 122 and 124 through windings 109 and 107. The signal across nodes 122 and 124 is coupled to nodes 115 and 120 through windings 105 and 103. However, because of the cross connection between winding 107 and winding 105, the signal at nodes 115 and 120 is out of phase with the signal at nodes 120 and 116. Thus, the signal at nodes 115 and 120 cancel the signal at nodes 120 and 116, resulting in no signal across nodes 115 and 116. Thus, a signal applied at the two wire bidirectional interface across nodes 113 and 114 appears across nodes 117 and 118 of the two wire receive signal interface, but not across nodes 115 and 116 of the two wire transmit signal interface.
Transformer based hybrid circuits have many disadvantages. They require specially wound transformers which are large, bulky, heavy and expensive. They require precise matching of the impedance of the matching network to the impedance of the telephone line. Furthermore, their frequency response may be limited and may prevent effective use for high speed data communications.
FIG. 2 illustrates a hybrid constructed with electronic components. This hybrid circuit avoids the need for specially wound transformers. The transmit signal interface includes node 211 and ground. Node 211 is coupled to the first terminal of resistor 204. The second terminal of resistor 204 is coupled through node 212 to the inverting input of operational amplifier (op amp) 201 and to the first terminal of resistor 203. The non-inverting input of op amp 201 is coupled to ground. The output of op amp 201 is coupled through node 213 to the second terminal of resistor 203, to the first terminal of resistor 206, and to the first terminal of resistor 207. The second terminal of resistor 206 is coupled through node 214 to the first terminal of resistor 205 and to the inverting input of op amp 202. The second terminal of resistor 207 is coupled to the non-inverting input of op amp 202, to the first terminal of resistor 208, and to node 217 of the two wire bidirectional interface. The other terminal of the two wire bidirectional interface is coupled to ground.
The output of op amp 202 is coupled to the second terminal of resistor 205 and to the first terminal of resistor 209. The second terminal of resistor 209 is coupled the first terminal of resistor 210 and to node 216 of the receive signal interface. The other terminal of the receive signal interface is coupled to ground. The second terminal of resistor 210 is coupled to ground.
Op amp 201 is configured as an inverting amplifier having a gain of two. A signal applied to the input of op amp 201 is amplified and applied to the voltage divider comprising resistor 207 and the impedance of the telephone line. Since resistor 207 has a value substantially equal to the impedance of the telephone line, and the telephone line impedance is much less than the value of resistor 208, the voltage divider drops half of the voltage across resistor 207 and half across the telephone line. The relatively high impedance of the telephone line leaves the voltage divider ratio substantially unaffected by resistor 208.
Op amp 202 is configured as a differential amplifier and produces an output equal to the input but inverted. The non-inverting input of op amp 202 amplifies one half of the output of op amp 201, but does not invert it. Since the signals are out of phase with one another, they cancel and op amp 202 provides zero output.
When a signal is applied to node 217, op amp 201 does not respond since the signal is coupled to its output. Since the signal is applied to the non-inverting input of op amp 202, the signal appears at the output of op amp 202 and at node 216 of the receive signal interface.
However, the rejection of the transmit signal provided by this circuit is limited by the extent to which resistor 207 matches the impedance of the bidirectional interface between node 217 and ground. Also, this circuit requires power supply voltages substantially higher than the level of the transmit signal. Thus, the amplitude of the transmit signal would be limited for low voltage power supply operation.